As a conventional photosensor, a device fabricated by forming a photodiode (PD) on a semiconductor substrate such as silicon (Si) is general. As for a solid-state imaging device, a flat solid-state imaging device is widely used, in which PDs are two-dimensionally arranged in a semiconductor substrate and a signal according to a signal charge generated by photoelectric conversion in each PD is read out through a CCD or CMOS circuit.
As a method for achieving a color solid-state imaging device, a structure in which a color filter transmitting only light at a specific wavelength for a color separation is disposed on the light incident surface side of the flat solid-state imaging device is common. In particular, a single-plate solid-state imaging device in which color filters transmitting blue (B) light, green (G) light and red (R) light, respectively, are regularly disposed on each of the two-dimensionally arranged PDs, is well known as a system widely used presently in a digital camera.
In this single-plate solid-state imaging device, since the color filter transmits only light at a limited wavelength, light not transmitted through the color filter is not utilized and the light utilization efficiency tends to be low. In addition, in recent years, manufacturing of a multipixel device is in progress, and the pixel size becomes smaller. As a result, the area of a photodiode part becomes smaller and problems such as the reduction in the aperture ratio and the reduction in the light collection efficiency arise.
In order to solve the disadvantages, a method is suggested stacking photoelectric conversion units capable of detecting light waves having different length in a longitudinal direction. For such a method, when limited to visible light, there is disclosed, for example, a method of forming a longitudinally laminated structure using a wavelength dependence on the absorption coefficient of Si, and separating colors by the difference in each depth (Patent Document 1), or a method of forming a first light receiving unit using an organic semiconductor, and second and third light receiving units composed of Si (Patent Document 2).
However, in these methods, there are disadvantages in that the color separation is insufficient because the absorption range in each light receiving unit has an overlapped portion in the depth direction of Si, and thus, spectral characteristics are poor. Further, as other solutions, a structure forming a photoelectric conversion film by amorphous silicon or an organic photoelectric conversion film on a substrate for reading-out signals is known as a means for raising aperture ratio.
Further, there are several publicly known examples for a photoelectric conversion using an organic photoelectric conversion film, an imaging device, a photosensor and a solar cell. The photoelectric conversion device using an organic photoelectric conversion film has a problem that needs to be resolved to enhance a photoelectric conversion efficiency or to reduce a dark current. Several methods have been disclosed for an improvement including introduction of a p-n junction or introduction of a bulk hetero-structure in the former case, and introduction of a blocking layer in the latter case.
In the case of making the photoelectric conversion efficiency high by the introduction of a p-n junction or a bulk hetero-structure, increase in dark current often becomes problematic. Further, since the degree of improvement in photoelectric conversion efficiency is different depending on the combination of materials, the ratio of optical signal amount to dark noise may not be increased compared to the ratio before introduction of these structures in some cases. When using these means, it is important which materials are combined, and in particular, when considering reduction in dark noise, it is difficult to achieve the reduction in dark noise with combination of materials which have already been reported.
Further, the kind of materials to be used and the film structure are main factors of a photoelectric conversion efficiency (exciton dissociation efficiency and charge transportability) and a dark current (carrier amount during a dark state and the like), as well as dominant factors of a signal response although it is not mentioned in the reports so far achieved. When using as a solid-state imaging device, it is necessary to satisfy all of a high photoelectric conversion efficiency, a low dark current and a high response speed, but, the organic photoelectric conversion materials and device structure of the kind have not been described in detail.
Although a photoelectric conversion film containing fullerenes is described in Patent Document 3, it is impossible to satisfy all of a high photoelectric conversion efficiency, a low dark current and a high response speed as described above with only fullerenes. Further, Patent Document 4 describes a device including a bulk hetero film composed of an organic matter having a specific structure and fullerenes as an organic photoelectric conversion film, but there has been no description about thermal stability and chemical stability of photoelectric conversion materials when fabricating the photoelectric conversion film.